Interpretive Summary: Columnaris disease, caused by the bacterium Flavobacterium columnare, is a costly disease of many commercially grown fish species including channel catfish. Despite its importance, few preventatives or therapies exist for this disease. In this study, we evaluated a type of clay called kaolin for the prevention of columnaris disease. Our research findings concluded that the addition of kaolin to the water significantly improved the survival of channel catfish that were experimentally infected with the disease and protected the gill from damage by the bacteria. We found that kaolin worked by binding to the bacteria, thereby preventing it from attaching to the fish and initiating disease. These results will aid fish producers by allowing for the better management of columnaris disease.

Technical Abstract:
Columnaris disease, caused by the bacterial pathogen Flavobacterium columnare, continues to be a major problem worldwide in aquaculture settings. Despite the far-reaching negative impacts of columnaris disease, safe and efficacious preventatives and curatives for this disease remain limited. In the present study, we evaluated the potential of kaolin (Al2Si205(OH)4), a type of clay, for the prevention of columnaris disease. Channel catfish fingerlings were experimentally challenged with a lethal dose of Flavobacterium columnare in untreated water or with water containing kaolin (1 g/L). Over the 7 d course of study, kaolin treatment led to significantly (P<0.001) improved survival (96.3%) as compared to untreated fish (78.8% survival). Histological examination of the gills revealed that kaolin-treated fish had substantially less gill damage than untreated controls. Quantitative PCR analysis of gill tissue revealed that kaolin significantly reduced Flavobacterium columnare adhesion (measured at 1 h post-challenge) and colonization (24 h post-challenge). Incubation of kaolin with Flavobacterium columnare in vitro demonstrated that kaolin reduced the number of Flavobacterium columnare cells in culture supernatants, presumably through the formation of physical complexes through adsorption. In summary, kaolin can improve survival, reduce gill pathologies, and reduce bacterial attachment to key tissues associated with columnaris disease in channel catfish by binding Flavobacterium columnare.